Transparent thermoplastic resins, especially, methacrylic resins have been employed for a wide variety of illumination uses because of their excellent light transmittance and mechanical characteristics. Recently, they are used as light guiding plates for back-lighting in display devices provided with illumination lamps. As the back-lighting systems, there are generally used two systems of so-called direct-light type in which the light guiding plate is interposed between a light source and a liquid crystal unit, and edge-light type in which a light source is provided at the side edge portions of the light guiding plate, and at present the edge-light type is mainly employed. Especially, with recent strong demands for high luminance display devices, large-sized display devices and thin display devices, luminous devices have been developed under the conception of making lighter, larger and thinner devices, and, among them, high luminance face luminous devices of edge-light type are especially desired.
Therefore, as to the light guiding plates used in light source devices, there have also been strongly demanded light guiding plates according to which loss of incident light entering from light source lamp disposed at the side edge portion is reduced as much as possible inside the light guiding plate and the incident light is allowed to efficiently outgo toward the outgoing face.
For meeting these demands, a plurality of technical disclosures have been made on methods for attaining high luminance by using light guiding plate. For example, JP-B-39-1194 discloses a method of obtaining a uniform luminous face by incorporating and dispersing light diffusing particles in a substrate of the light guiding plate. Furthermore, JP-A-4-145485 discloses to attain a high luminance by using a light scattering plastic material containing fine particles different in refractive index as a light guide. Moreover, JP-A-2000-113708 discloses a method for attaining high luminance by dispersing fine particles having hollow structure and differing in refractive index in a light guiding plate.
In the above prior art, fine particles are incorporated, but optimization of the kind and the average particle diameter of the fine particles is not made, and the effect to improve luminance is small, and thus in case the display devices are made larger and thinner, sufficiently high luminance has not yet been obtained.
An object of the present invention is to provide a light guiding plate suitable for display devices used for office automation apparatuses such as personal computers, word processors, etc., and various monitors displaying image signals such as panel monitors, television monitors, etc., display devices used for illuminators for indoor or outdoor space, and signs.
Another object of the present invention is to provide a method for stable production of a resin composition for the light guiding plate.